Flexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of substances particularly those that are soft and easily deformed, such as paper, paperboard stock, corrugated board, polymeric films, fabrics, plastic films, metal foils, and laminates. Course surfaces and stretchable polymeric films can be economically printed by the means of flexography.
Flexographic printing plates are sometimes known as “relief printing plates” and are provided with raised relief images onto which ink is applied for application to the printing substance. The raised relief images are inked in contrast to the relief “floor” that remains free of ink in the desired printing situations. Such printing plates are generally supplied to the user as a multi-layered article having one or more imagable layers coated on a backing or substrate. Flexographic printing can also be carried out using a flexographic printing cylinder or seamless sleeve having the desired raised relief image. These flexographic printing cylinder or sleeve precursors can be “imaged in-the-round” (ITR), either by using a standard photomask or a “laser ablation mask” (LAM) imaging on a photosensitive plate formulation, or by “direct laser engraving” (DLE) of a plate precursor that is not necessarily photosensitive.
Generally, flexographic printing plates are produced from a photosensitive resin. A photo-mask, bearing an image pattern is placed over the photosensitive resin sheet and the resulting masked resin is exposed to light, typically UV radiation, to crosslink the exposed portions of the resin, followed by developing treatment in which the unexposed portions (non-crosslinked) of the resin are washed away with a developing liquid. Recent developments have introduced the CTP (computer-to-plate) method of creating the black mask for the photosensitive resin. In this method, a thin (generally 1-5 μm in thickness) light absorption black layer is formed on the surface of the photosensitive resin plate and the resultant plate is irradiated imagewise with an infrared laser to ablate portions of the mask on the resin plate directly without separately preparing the mask. In such systems, only the mask is ablated without ablating the photosensitive plate precursor. Subsequently, the resultant plate is imagewise exposed to light through the ablated areas of the mask, to crosslink (or harden) the exposed portions of the photosensitive resin, followed by developing treatment in which the unexposed portions (uncrosslinked) of the resin and the remaining black mask layer are washed away with a developing liquid. Both these methods involve developing treatment that requires the use of large quantities of liquids and solvents that subsequently need to be disposed of. In addition, the efficiency in producing plates is limited by the additional drying time of the developed plates that is required to remove the developing liquid and dry the plate.
U.S. Pat. No. 5,719,009 (Fan) describes elements having an ablatable layer disposed over photosensitive layer(s) so that after image ablation, UV exposure of the underlying layer hardens it while non-exposed layer(s) and the ablatable mask layer are subsequently washed away.
DuPont's Cyrel® FAST™ thermal mass transfer plates are commercially available photosensitive resin plate precursors that comprise an integrated ablatable mask element and require minimal chemical processing, but they do require thermal wicking or wiping to remove the non-exposed areas. These also require extensive disposal of liquid polymeric waste and some drying of the processed (developed) plates.
There remains a need for a totally processless method of producing flexographic printing plates with high throughput efficiency. A method for forming a relief pattern on a printing element by directly engraving (DE) with a laser is already used to produce relief plates and stamps. However, the requirement of relief depths in excess of 500 μm challenges the speed at which these flexographic printing plate precursors can be imaged. In contrast to the laser ablation of the CTP mask layers atop the photosensitive resin, which only requires low energy lasers and low fluence, the DE of laser ablatable flexographic printing plates requires higher energy lasers and higher fluence. In addition, the laser ablatable, relief-forming layer becomes the printing surface and must have the appropriate physical and chemical properties needed for good printing. The laser engraveable black mask layer is washed away during the development and is not used during the printing.
Flexographic printing plate precursors used for infrared radiation (IR) laser ablation engraving must comprise an elastomeric or polymeric composition that includes one or more infrared radiation absorbing compounds. When the term “imaging” is used in connection with “laser engraving”, it refers to ablation of the background areas while leaving intact the areas of the element that will be inked and printed in a flexographic printing station or press.
Commercial laser engraving is typically carried out using carbon dioxide lasers. While they are generally slow and expensive to use and have poor beam resolution, they are used because of the attractions of direct thermal imaging. Infrared (IR) fiber lasers are also used. These lasers provide better beam resolution, but are very expensive. However, it would be preferable to use infrared (IR) diodes for infrared radiation engraving that have the advantages of high resolution and relatively lower cost so that they can be used in large arrays. In any case, it would be preferable to use higher power laser that approach deposing the energy adiabatically. IR laser engraveable flexographic printing plate blanks having unique engraveable compositions are described in WO 2005/084959 (Figov).
Direct laser engraving is described, for example, in U.S. Pat. Nos. 5,798,202 and 5,804,353 (both Cushner et al.) in which various means are used to reinforce the elastomeric layers. The reinforcement can be done by addition of particulates, by photochemical reinforcement, or by thermochemical hardening. U.S. Pat. No. 5,804,353 describes a multilayer flexographic printing plate wherein the composition of the top layer is different from the composition of the intermediate layer. Carbon black can be used as a reinforcing agent and can be present in both layers. There is no description how this component can impact the engraving process and resulting flexographic printing plate and there is no specific connection contemplated between this and laser ablation efficiency. This patent provides no guidance as to the relative levels of carbon black in each of the layers relative to other layers.
There are a number of elastomeric systems that have been considered for construction of laser engraveable flexographic printing plates. There are many systems that include various IR absorbing particles. However, these systems suffer from poor engraving efficiency when it is desired to ablate several hundred microns into the element and no guidance is provided as to the optimal loading of IR absorbers with respect to the amount of resin or to the effect of concentration of the IR absorber on laser engraving efficiency. When a low concentration of IR absorbing compounds is incorporated into the element, there is either not enough absorption of energy to cause ablation, or there is excessive liquefaction of the element with little ejection of material. Even when ejection occurs, the presence of excess liquefaction, or viscous un-ejected material can be difficult to remove from the ablated plate. This can also cause problems such as imprecise edges of the imaged features of the relief pattern and the adherence of molten polymer to the surfaces and/or sides of the relief pattern. This ultimately will interfere with image feature quality and printing quality. Further, when large amounts of liquid or viscous material are generated during the laser ablation and are ejected, this debris can stain the optical parts of the laser engraving apparatus, such as the lens, and causes problems with the apparatus. When a high loading of IR absorbing compound is used, there is a decrease in the laser penetration depth due to the Beer-Lambert law of absorbance, and poor ablation efficiency. Another disadvantage to high incorporation of the IR absorbing compounds is that many such compounds, including carbon black, also absorb in the UV region and thus would block any UV radiation that could be used to photochemically crosslink or cure the element composition. There remains a need for laser ablatable compositions that provide increased engraving efficiency so as to increase plate imaging speed and throughput.